(a) Field of the Invention
This invention relates to exhaust gas purification catalysts for reducing NOx (nitrogen oxides) emissions from vehicle engines.
(b) Description of the Related Art
Lean-NOx catalysts are known as catalysts for purifying exhaust gases from vehicle engines. The lean-NOx catalysts contain a NOx storage component such as barium (Ba), and a catalytic metal such as platinum (Pt). In an oxygen-rich atmosphere of exhaust gas (during engine operation at lean air-fuel ratios) where the oxygen concentration in the exhaust gas is high, the lean-NOx catalysts store NOx in the exhaust gas with the NOx storage component. When the oxygen concentration in the exhaust gas drops (during engine operation in the vicinity of the ideal air-fuel ratio or at rich air-fuel ratios), the catalysts release NOx stored in the NOx storage component and reduce the released NOx to N2 with the catalytic metal. For NOx reduction reaction, HC (hydrocarbon) and CO (carbon monoxide) in the exhaust gas are used as reductants. Therefore, HC and CO are removed from the exhaust gas by oxidation concurrently with the removal of NOx by reduction reaction with HC and CO.
In such a lean-NOx catalyst, a catalytic metal such as Pt and oxygen excessively supplied during lean burn operation are utilized for oxidation of NO in exhaust gas to NO2. This oxidation makes it easy that NO is stored into the NOx storage component.
Japanese Unexamined Patent Publication No. H08-281106 discloses a technique for a lean-NOx catalyst in which all or part of precious metal, such as Pt, is supported by impregnation on ceria serving as an oxygen storage component to accelerate the oxidation of NOx, thereby enhancing the NOx storage effect of the NOx storage component.
Japanese Unexamined Patent Publication No. H11-169712 discloses a lean-NOx catalyst in which first (upper) and second (lower) catalytic layers are formed as a catalytic coating on a honeycomb support, the first catalytic layer has a structure in which a mixture of alumina and cerium dioxide serving as an oxygen storage component are used as a support material and Pt and Ba are supported on the support material, and the second catalytic layer has a structure in which a mixture of cerium dioxide, zirconium dioxide or cerium (Ce)-zirconium (Zr) mixed oxide and alumina are used as a support material and Rh is supported on the support material.
Further, the applicant has previously applied a patent application for a mixed oxide containing Ce, Zr and Rh and a mixed oxide containing Ce, Zr, Nd and Rh which are useful as catalytic materials. The patent application is published as Japanese Unexamined Patent Publication No. 2004-174490. This document discloses that Rh-containing Ce-based mixed oxides of this kind are prepared by coprecipitation, and that if comparison is made between a mixed oxide containing Ce, Zr, Nd and Rh and a substance obtained by post-supporting Rh on a mixed oxide containing Ce, Zr and Nd, the former has a higher oxygen storage capacity (amount of oxygen storage and rate of oxygen storage) and a higher heat resistance.
As described above, in the known technique for forming a lean-NOx catalyst, a catalytic metal such as Pt is supported on a support material such as cerium, for example, by using impregnation to bring a Pt solution into contact with the support material. In this case, however, Pt is easily sintered by heat, which deteriorates the catalyst's functions of oxidation of NO to NO2, NOx reduction and in turn HC and CO oxidations.
Further, since cerium dioxide employed as an oxygen storage component for lean-NOx catalysts has a poor heat resistance, it deteriorates with long-time use of the catalysts to decrease its amount of oxygen storage. When Ce-based mixed oxides containing Ce, for example, Ce—Zr mixed oxides, are employed as oxygen storage components instead, they have high heat resistance but their amounts of oxygen storage and release are small.
Therefore, even when either cerium dioxide or Ce—Zr mixed oxide is used for a lean-NOx catalyst, the catalyst exhibits a poor HC oxidation capacity at low temperatures, so that it cannot take full advantage of the function of catalytic metal to promote HC oxidation and in turn deteriorates its NOx removal performance.
Furthermore, as in the lean-NOx catalyst disclosed in Japanese Unexamined Patent Publication No. H11-169712, when a catalytic coating including a plurality of catalytic layers is formed on a support, the oxygen storage component is generally provided in the lower catalytic layer (the catalytic layer closer to the cell wall) in order to protect it against high-temperature exhaust gas. Since, however, the diffusive flow of exhaust gas through the lower catalytic layer is small, the lower catalytic layer cannot sufficiently utilize highly active oxygen just released from the oxygen storage component for the purpose of HC oxidation, which prevents efficient NOx reduction.